In mass spectrometers used for proteome analysis or the like, separation of multiple charge ions is very important. In electrospray ionization, most of noise ions are singly charged, whereas peptide ions tend to be multiply charged. Accordingly, technologies are very important for effective separation of only multiple charge ions from singly charged ions. Information on the charge number is obtained by analyzing mass spectra provided as a result of measurements with high resolution and less spectrum duplication. A sample previously subjected to a simple pretreatment, however, contains multiple components, and spectra thereof are superimposed on one another. This makes it difficult to identify the multiple charge ions and the singly charged ions by means of software. To approach the above-mentioned problem, the U.S. Pat. No. 2002/0175279 discloses a method for achieving charge separation by means of hardware. In the method disclosed, collision of gas molecules inside a linear trap allows ions to be cooled to the thermal temperature. Then, potential on one or both sides of an end lends in the linear trap is decreased to be a potential D of 0.1 to 1 V with respect to an offset potential of a linear trap section. At this time, a trap potential of the singly charged ions is the potential D, while a potential of n-charged ions is a potential nD. In contrast, kinetic energy of ions is maintained to be approximately thermal temperature energy (kT) regardless of the charge under cooling of the ions. Since the ion energy has a Maxwell-Bolzmann distribution, ions are ejected outside the trap in order from low to high charge, wherein the lower charged ions form a lower potential than the multiple charge ions. After this processing, mass spectrometry is carried out in the linear trap. Alternatively, ions may be introduced into a time-of-flight mass spectrometer so as to perform the mass spectrometry. During this time, a collision gas chamber or the like may be provided to perform MS/MS analysis or the like, as disclosed in the above document.
In the linear trap, separation of ions based on the mass-to-charge ratio (m/n, m: mass, n: charge number) has hitherto been carried out using a supplemental AC voltage, as disclosed in, for example, the U.S. Pat. No. 5,420,425. According to this document, a harmonic potential is formed radially by a RF voltage. A supplemental AC voltage which resonates with the harmonic potential is applied to between electrodes opposed to each other to radially eject the ions with the specific mass-to-charge ratio.
Another method for ion separation based on the mass-to-charge ratio in the linear trap is disclosed in the U.S. Pat. No. 6,177,668. According to this document, a harmonic potential is formed radially by a RF voltage. A supplemental AC voltage which resonates with the harmonic potential is applied to between electrodes opposed to each other, or between quadrupole rods and end lenses to axially eject the ions with the specific mass-to-charge ratio.
A further method for ion separation based on the mass-to-charge ratio (m/n) in the linear trap is disclosed in the U.S. Pat. No. 5,783,824. Wing electrodes are inserted into between multipole rods to form a harmonic potential on an axis. A supplemental AC voltage which resonates with the harmonic potential is applied to between the wing electrodes to axially eject the ions with the specific mass-to-charge ratio.
Ion mobility separation in the mass spectrometry is disclosed in the U.S. Pat. No. 5,905,258. Ions ejected pulsely from an ion source or an ion trap are subjected to a constant DC electric field under gas pressure of approximately 10 mTorr. Since the velocities of ions accelerated by the electric field are different from each other, separation of the ion mobility is performed in an acceleration area of the DC electric field. Timings at which the ions reach a mass spectrometry section are different due to the ion mobility thereof, which can facilitate the separation.